The first simple molecular assembler is likely to be a macroscale
device, perhaps a modified SPM system as was being pursued by Zyvex starting
in 1998 (Section 4.14). Multiple SPM heads could be equipped
with a small number of nanoscale tool tips. In some scenarios, nanoparts fabricated
using either bulk chemistry or positional mechanosynthesis techniques would
be inspected and selected by the SPM, then assembled one by one into working
nanomachines (e.g., the desired useful nanoscale products). Such assembly operations
will be very slow, because the placement of each new component may require simultaneous
rotations and translations of large macroscale SPM components. Assembly time
may scale roughly linearly with assembler size [208]
because smaller assembler components moving at a given velocity need to travel
less distance to accomplish a given physical operation, consuming less time
and energy per physical operation. Hence an important early developmental goal
would then be to design and fabricate nanoscale molecular assemblers whose manipulatory
components are closer in size to the scale of the parts which must be assembled.

In 1996, Freitas [2679]
informally proposed the “Efficient Replicator Scaling Conjecture”
which holds that “the most efficient replicator will operate on a substrate
consisting of parts that are approximately of the same size scale as the parts
with which it is itself constructed. Hence a robot made of ~1 cm parts will
operate most efficiently in an environment in which ~cm-scale parts (of appropriate
types) are presented to it for assembly. Such a robot would be less efficient
if it was forced to build itself out of millimeter or micron-scale parts, since
the robot would have to preassemble these smaller parts into the 1-cm parts
it needed for the final assembly process. Similarly, input parts much larger
than 1 cm would have to be disassembled or milled down to the proper size before
they could be used, consuming additional time, information, and physical resources
– also reducing replicative efficiency. If this conjecture is correct,
then it follows that to most efficiently replicate from an atomic or molecular
substrate, you would want to use atomic or molecular-scale parts – that
is, nanotechnology.”

This conjecture seems broadly consistent with Drexler’s
scaling analysis of an exemplar manufacturing system architecture using 10 stages
of convergent assembly in which product scale is commensurate with mechanism
scale at all but the earliest stages of input ordering and reagent preparation
involving simple molecular inputs (Nanosystems [208],
Table 14.1; see also Section 4.9.3). It also seems consistent
with Merkle’s analysis of the convergent assembly approach to molecular
manufacturing (Section 5.9.4) using a progression of
manufacturing stages at different scales [213].
As Fearing [1581] noted in connection with
the efficient construction of microdevices: “There are many advantages
to shrinking robots and mechanical actuators to the same size as the parts to
be manipulated.....”